The present invention relates to a sitting device, adapted to be used in two different sitting postures, viz. on one hand normal sitting posture, in which one is sitting on a substantially horizontal seat with the soles of the feet against the floor, and on the second hand a kneeling-like sitting posture, in which one is sitting on a forwardly sloping seat and rests the lower parts of the legs and/or the knees against a separate knee support, which is pivotally mounted in one end each of a pair of supporting arms, which have their other ends pivotally mounted in links of a link system, said links being arranged in pairs and attached to the frame of the sitting device pivotable in a vertical plane for displacement of the knee support between a normal position below the seat and an advanced position in front of and below the seat.